1. Field of the Invention
The field of the invention is that of photodetectors, and more precisely so-called quantum well photodetectors operating in the medium infrared, known by the acronym QWIP standing for Quantum Well Infrared Photodetector.
2. Description of the Prior Art
In order to improve the performance of the detectors it is necessary to optimize their detectivity, i.e. the ratio between the response of the detector and the noise. When the electronic noise is negligible, the total noise is proportional to the square root of the area of the electrically active zone of the detector. The response, for its part, is proportional to the light flux absorbed. With a constant absorbed flux, it is therefore necessary to reduce the area of the absorption zone in order to improve the detectivity. Similarly, the dark current is proportional to the area of the active zone. In imaging systems in which the dark current limits the performance of the readout circuit, it is therefore also important to reduce the size of the area of the active zone in order to increase performance.
When a single detector is used, in order to reduce the area of the active zone while conserving the flux it is possible, for example, to focus the light onto a detector of size D using a lens with a diameter φ slightly greater than D, in which case an improvement of the signal/noise ratio by a quantity equal to √{square root over (φ/D)} is obtained. The signal/dark current ratio is also improved by a factor φ/D. This solution is not applicable per se to a matrix of detectors.